The disclosed invention generally relates to a microwave stripline coupler, and is more particularly directed to a microstrip coupler structure which can be made with processes having loose tolerances including thick film and low temperature co-fired processes.
Microstrip couplers are 4-port microwave integrated circuit elements utilized for directing microwave energy. In a very basic form, a microstrip coupler comprises two closely spaced, edge by edge parallel microstrip transmission lines having a common ground plane. The ends of one of the microstrips comprise an incident port and a direct port, while the ends of the other comprise a coupled port and an isolated port, with the coupled port being next to the incident port. Ideally, microwave power applied to the incident port is directed to the direct port and the coupled port only, but in practice some power is coupled to the isolated port. The relative amount of power available at the coupled port can be characterized by coupled port insertion loss, wherein a numerically greater dB figure means less power is coupled to the coupled port, which is achieved by a greater spacing between the microstrips. Thus, a coupler characterized by 10 dB of coupling has a greater separation between the microstrips than a coupler having 3 dB of coupling; and the 10 dB coupler couples less power to the coupled port than the 3 dB coupler.
Microstrip couplers are designed with different coupled port losses for different applications. An important coupler is the 3 dB coupler which divides the incident power so that half is available at the direct port and half at the coupled port.
Microwave circuit elements, particularly those designed for shorter wavelengths, are small and therefore candidates for integrated circuit manufacturing techniques. As to parallel microstrip couplers, however, the close separation and stringent tolerances required for tight coupling are not readily achieved with photolithographic techniques. Other more complex structures have been devised for implementation with photolithographic techniques, including an interdigited coupler (sometimes called a Lange coupler after its originator) which includes three or more parallel transmission lines, wherein non-adjacent lines are connected in parallel at their respective ends, and sometimes additionally at the centers of the lines, by wire bonds or conductive bridges.
Important considerations with the interdigited coupler structure include complex manufacturing procedures, low yield, wire bonding labor and size. Moreover, it is not amenable to the reduced tolerances of thick film processing techniques which can be utilized to produce other microwave circuit elements at significantly lower cost.